A couple of weeks back, Charlie-in-my-writing-group was recruiting a semipro SF writer who'd just moved to town to join our workshop. In his email to the new writer, he described me as "prolific".
No, I'm not prolific. Not by far. Take Gary A. Braunbeck, for instance -- he's prolific. 200 published stories, three collections, four novels, and five contracts for more. He's one of the folks the book publishers call up and say, "Hey Gary, we're 10,000 words short for our new anthology. Can you send us a new story that length in four days?" And he sits down and cranks out a good, publishable novella in less than a week. And his output is nothing compared to the likes of Stephen King or Isaac Asimov -- Asimov could turn out a 50,000-word novel in a week. That's prolific.
What I am is persistent. I get a rejection, I revise my story (or not) and send it right back out again. I don't stop until my stories find a home. Sometimes they sell quick, sometimes it takes years and lots of revision -- but I don't stop until they sell. It's because I can't shake the notion that if a story never sees publication -- be that publication large or small -- it's useless. To me, an unpublished story is a story that doesn't really exist.
Lots of fiction writers -- the bulk of the folks I went to the Clarion workshop with, unfortunately -- hit a couple of markets, get their rejections, and quit, deciding their work is no good.
Sure, it does no good to keep sending out something that's unpublishable -- if I've gotten, say, ten rejections on a piece, I take a long, hard look at it. But three rejections? Four? Five? Pshaw. 'Taint nothing.
The thing is, you have to develop the ability to objectively evaluate your own work -- and then you have to trust your own instincts. You have to have a little faith in yourself and your work.
And you have to expect rejections. Expect the worst and it won't sting you. Easier said than done, I know.